1. Field of the Invention
The present invention relates to a color processing method and image forming apparatus for creating a color separation table used to separate image data into data of a plurality of colorants.
2. Description of the Related Art
Along with recent improvement in the quality of formed images in image forming apparatuses such as color printers which visualize color images on print media, standards of user demands for media are rising. To meet these standards, one printer needs to handle various types of media, and output a high-quality image on any medium.
Generally, a color printing system using a color printer or the like executes a color separation process to separate color image data to be formed into the respective amounts of colorants (inks or toners) in the color printer. The color separation process is performed based on a color separation table created in advance. However, an optimal color separation process changes depending on the type of medium. Therefore, a plurality of color separation tables are preferably prepared to implement optimal color separation processes for respective media types.
According to a conventional technique, the color separation table is manually created, so designing color separation tables for various types of media greatly prolongs the design period. To quickly create color separation tables for various types of media, a technique for automatically generating a color separation table relating to an arbitrary type of medium is required.
In general, items in the color separation table have a trade-off relationship: if the color gamut is widened, the tonality is impaired depending on the colorant to be used. Thus, the user needs to adjust an automatically generated color separation table accordingly.
In a generated color separation table, when the illuminant changes, the color gamut may change or the tonality may suffer. Thus, a color separation table needs to be created for each illuminant. It is desirable to allow the user to adjust a color separation table corresponding to an illuminant by using a user interface which visualizes the color separation table in the Lab space.
As described above, when automatically generating a color separation table corresponding to a medium, the color separation table needs to be optimized in accordance with the illuminant. In other words, a tool (user interface) is necessary to allow the user to arbitrarily adjust a created color separation table. Creating such a user interface requires a color prediction technique capable of simulating a mixed color development characteristic without printing a color patch on a medium or measuring the color.
According to the color prediction technique of simulating a color development characteristic upon mixing colorants, it is necessary to divide the colorant space into grids, print all grid points (to be referred to as primaries hereinafter), and measure the color. However, some primaries cannot be printed owing to the restriction on the total colorant amount on a medium. For such an unprintable primary, there is proposed a technique of making primaries strictly observe the total colorant amount restriction by simple linear correction (see, for example, Japanese Patent Laid-Open No. 2003-334934). However, when a primary is linearly corrected in accordance with the total colorant amount restriction, the linear relationship with peripheral primaries is lost, decreasing the color predictability. In addition, if the number of colorants for use increases, the number of primaries and the printing load also increase.
The color prediction process uses a color reproducibility look up table which defines the spectral value of each primary with respect to a medium for use. The color reproducibility table looked up in the color prediction process will be explained with reference to FIGS. 14 and 15.
FIG. 14 is a flowchart showing a conventional color reproducibility table creation process.
In step S1401, parameters are input. The parameters are data such as total colorant amount restriction information of a medium, and the number and pitch of grids in an ink space. In step S1402, after all primaries are generated, primaries which are printable because the total colorant amount falls within the total colorant amount restriction are generated as patches. In step S1403, the patches generated in step S1402 are printed on a medium to measure the color. In step S1404, the color measurement results are input.
In step S1405, the color measurement values of primaries which are unprintable because the total colorant amount exceeds the total colorant amount restriction need to be estimated using the color measurement values. By these steps, the color measurement values, that is, the spectral values of all primaries can be obtained.
The created color reproducibility table enables color prediction at an arbitrary point in the color space including primaries. More specifically, when the signal value of a point subjected to prediction is input, the spectral value of the point can be output by executing color prediction by a known cellular Neugebauer process based on the color measurement values of primaries around the point in the color reproducibility table.
FIG. 15 is a view showing an example of printable and unprintable primaries when two, cyan (C) and magenta (M) inks are used. In FIG. 15, frame lines between four points W, C, M, and C+M are divided by the number of grids (three in this example) to obtain primaries. In this example, each frame line is equally divided into three at points 0, 85, 170, and 255. In FIG. 15, the symbol ● represents a printable primary falling within the total colorant amount restriction indicated by a dotted line, and each printable primary is formed as a color prediction patch. The symbol ◯ represents an unprintable primary exceeding the total colorant amount restriction. An unprintable primary attains an appropriate spectral value by the color prediction process using the above-mentioned color prediction patches.
However, the color reproducibility table creation process suffers from the following problems. The calculation for estimating a primary assumes that created patches are properly printed. If the patch of a printable primary has not been printed appropriately for some reason, the reliability of the estimation result of an unprintable primary becomes poor. In this case, the precision of a color reproducibility table created based on estimated primaries decreases. The precision of a color separation table created by color prediction looking up the color reproducibility table also decreases.